Perhaps they want to discourage people from only running Garuda for weapons. This way once you're 7/7 on weapons with Garuda, you have a reason to go back and do Ifrit and Moogle (which will help people who haven't had a chance to access that content yet) instead of just spamming Garuda until next update. I imagine whenever they release the next avatar they will let people convert their Garuda weapons as well.